1. Field of the Invention
The present invention relates generally to the field of photonic crystals; and, more particularly, to a photonic crystal waveguide apparatus having a resonant stub tuner.
2. Description of Related Art
Photonic crystals (PC) are periodic dielectric structures which can prohibit the propagation of light in certain frequency ranges. More particularly, photonic crystals have spatially periodic variations in refractive index; and with a sufficiently high refractive index contrast, photonic bandgaps can be opened in the structure""s optical transmission characteristics. (The term xe2x80x9cphotonic bandgapxe2x80x9d as used herein and as is commonly used in the art is a frequency range in which propagation of light through the photonic crystal is prevented. In addition, the term xe2x80x9clightxe2x80x9d as used herein is intended to include radiation throughout the electromagnetic spectrum, and is not limited to visible light.)
It is known that introducing defects in the periodic structure of the photonic crystal allows the existence of localized electromagnetic states that are trapped at the defect site, and that have resonant frequencies within the bandgap of the surrounding photonic crystal material. By providing a region of such defects extending through the photonic crystal, a waveguiding structure is created which can be used in the control and guiding of light.
A photonic crystal which has spatial periodicity in three dimensions can prevent the propagation of light having a frequency within the crystal""s bandgap in all directions; however, the fabrication of such a structure is technically challenging. A more attractive alternative is to utilize a 2-dimensional photonic crystal slab that has a two-dimensional periodic lattice incorporated within it. In a structure of this sort, light propagating in the slab is confined in the direction perpendicular to a major surface of the slab via total internal reflection, and light propagating in the slab in directions other than perpendicular to a major surface is controlled by the properties of the photonic crystal slab. A two-dimensional photonic crystal slab has the advantage that it is compatible with the planar technologies of standard semiconductor processing; and, in addition, the planar structure of the slab makes an optical signal in a waveguide created in the slab more easily accessible to interaction. This provides the additional advantage that the structure is susceptible to being used to create active devices.
Both theoretical and experimental work have demonstrated the efficient guidance of light in a two-dimensional photonic crystal slab waveguide device (see xe2x80x9cDemonstration of Highly Efficient Waveguiding in a Photonic Crystal Slab at the 1.5 xcexcm Wavelengthxe2x80x9d, S. Lin, E. Chow, S. Johnson and J. Joannopoulos, Opt. Lett. 25, pp. 1297-1299, 2000). Furthermore, experimental work is also beginning to demonstrate the capability of fabricating such devices that are able to propagate light with a high degree of efficiency; and it is only a matter of time before the fabrication of excellent photonic crystal waveguide devices become routine. As a result, there has already been some investigation into potential applications for interacting with the guided optical modes of the waveguide device. Such applications which have previously been discussed include static (fixed wavelength) or tunable channel drop filters, and tunable resonant microcavity defects (see U.S. Pat. No. 6,058,127).
An optical modulator and an optical switch which are based upon tunable resonant microcavity defects have also been described in the literature. In these devices, a waveguide structure is described which has a one-dimensional periodic dielectric photonic crystal along the propagation axis. This photonic crystal structure generates a frequency stop band in the transmission characteristic of the waveguide. Additionally, a defect is introduced in the periodic structure causing a localized resonant mode to occur within the frequency stop band of the waveguide. This resonant mode allows tunneling from one side of the defect to the other when the guided mode of the waveguide has a frequency which precisely matches that of the defect resonance. In this way, light can propagate down the waveguide, tunnel through the resonant defect and continue down the waveguide with a relatively high efficiency.
It is further described how the dielectric constant of the resonant defect region of the device can be changed via current injection or optical non-linearities so as to make the resonant frequency tunable, and thus provide a narrow band optical modulator or a tunable narrow passband switch.
The present invention provides a photonic crystal waveguide apparatus for controlling the transmission of light in a waveguide of the apparatus.
An exemplary photonic crystal waveguide apparatus according to the present invention may comprise a photonic crystal, a waveguide in the photonic crystal which is capable of transmitting light having a frequency within a bandgap of the photonic crystal, and a resonant stub connected to the waveguide to control light in the waveguide.
According to a first embodiment of the invention, the resonant stub comprises a resonator region and a connecting channel connecting the resonator region and the waveguide; and the resonator region and the connecting channel cooperate to control transmission characteristics of light in the waveguide. In particular, the resonator region and the connecting channel function to create a frequency range, commonly referred to as a xe2x80x9ctransmission zeroxe2x80x9d, within the bandgap of the photonic crystal at which light that is otherwise capable of being transmitted by the waveguide is prevented from being transmitted. The frequency of the transmission zero is a function of the resonant frequency of the resonator region, while the width of the transmission zero is a function of parameters of the connecting channel. Accordingly, by controlling parameters of the resonator region and of the connecting channel, the frequency of the transmission zero and its width can be controlled.
In accordance with a second embodiment of the invention, the waveguide comprises a region of first defects in a periodic lattice of the photonic crystal which extends through the photonic crystal; and the connecting channel comprises one or more second defects in the periodic lattice which are connected to the waveguide and which extend angularly from a sidewall of the waveguide. The resonator region comprises a region in the photonic crystal in which the periodic lattice has been modified in an appropriate manner to define a resonator chamber.
According to a third embodiment of the invention, the periodic lattice of the photonic crystal comprises an array of posts, and the waveguide is created by omitting a single line of the posts. The connecting channel is created by omitting two additional posts in the lattice to define a short channel which is connected to the waveguide and which extends perpendicularly from a sidewall of the waveguide. The resonator region comprises a generally square region having a 3xc3x973 sub-array of posts which are larger in diameter than the other posts in the lattice. By controlling parameters of the resonator region, such as the number of posts in the region and the size of the posts; the resonant frequency of the resonator region, and, hence, the spectral position of the transmission zero can be effectively controlled. By controlling one or more parameters of the connecting channel, such as its length and width, and, in embodiments in which the connecting channel includes posts, the presence, absence and modification of posts in the connecting channel; the spectral widths of the transmission zero can be controlled.
According to a fourth embodiment of the present invention, the apparatus includes a tuner for tuning parameters of the resonant modes of the resonant stub. The tuner may comprise a dielectric constant tuner for tuning the dielectric constant of the material comprising the posts in the resonator region. The dielectric constant tuner can be an electronic tuner for tuning the dielectric constant using, for example, the charge carrier effect or the electro-optic effect. Alternatively, the tuner can be an optical tuner for tuning the dielectric constant using, for example, the photorefractive effect. The tuner permits the waveguide transmission to be effectively tuned within a range extending from essentially zero transmission to substantially the full transmission value for the photonic crystal waveguide apparatus. This capability enables a wide variety of photonic crystal devices to be realized which can be utilized in essentially any application in which a tunable optical filter can be used including, but not limited to, on/off switches and optical modulators.
A photonic crystal waveguide apparatus according to the present invention operates as a tunable notch filter, where the action of the resonator is as a purely reflective agent causing the propagation characteristic of the waveguide to have a transmission zero at specified, tunable frequencies. The apparatus of the present invention is, accordingly, fundamentally different from other tuned photonic crystal devices that are known in the art.
Yet further advantages and specific features of the present invention will become apparent hereinafter in conjunction with the following detailed description of presently preferred embodiments of the invention.